Logarithmic count rate or frequency meter



June 18, 1957 G. A. MoRToN Erm.

LOGAEITHMTG couNT RATE 0R FREQUENCY METER Filed 'April 27, 1953- Mw mm.

2,796,533 Patented June 18, 195'7.v

2,796,533 LOGARITHMIC COUNT RATE R FREQUENCY METER George A. Morton and Kenneth W. Robinson, Princeton, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application Aprii 27, 1953-, Serial No. 351,218 The terminal fifteen years of the term of the patent to granted has been disclaimed 7 Claims. (Ci. Z50-214) This invention relates generally to electrical measuring instruments and particularly to a new and improved count rate or frequency measuring meter which operates over awide counting or frequency range.

Previously a number of different instruments have been devised for use as count rate or frequency measuring instruments. Many of these instruments are utilized in connection, with scintillation counters or employ relatively expensive ionization tubes such as Geiger tubes in which a gas is ionized each time a pulse to be counted is applied to the tube. Other types of apparatus employ conventional circuitry not requiring special purpose tubes. Some of both of the above types of apparatus utilize indicator scales which are linear reading. Since these scales are essentially linear it often is necessary to provide scale switching or other means which permit the apparatus to operate over an appreciable count or frequency range.

lt is an object of the present invention to provide an improved count rate or frequency measuring instrument.

Another object of the invention is to provide an improved count rate or frequency measuring instrument in which a logarithmic indication is afforded.

Another object of the invention is to provide an improved count rate or frequency measuring instrument capable of measuring pulse count rates over a range of the order of l pulses per second and capable of measuring signal frequencies over a like range.

A further object is to utilize the phenomena of the logarithmic distribution in the height of pulses derived in response to single electrons entering a secondary emission multiplier for providing a logarithmic instrumentl of the above type.

According to the present invention, the foregoing objects and advantages may be accomplished as follows. In a typical embodiment of the invention, randomly ocA curring input pulses (the average rate of occurrence of which is to be determined) are applied to one input circuit of a dual-input bistable multivibrator. A light source irradiates the photocathode of a photomultiplier tube to liberate electrons therefrom. These electrons enter the electron multiplier section of the tubeV and pro.- duce at its output pulses having a logarithmic pulse height distribution. These pulses are applied to the remaining input circuit of the multivibrator. then derived from the multivibrator which signal is averaged and utilized to control the threshold which pulses must attain to trigger the bistable device so that aV con-V stant ratio of multiplier and input pulses trigger the multi'J vibrator. The averaged output signal Voltage is a measure of the logarithmic count rate.

The invention will be described in greater detail with reference to the accompanying drawing in which:

Figure l is a schematic circuit diagram, in block form, of a logarithmic count rate or frequency measuring instrument, according to the invention; and,

Figure 2 is a circuit diagram of a Variable bias dis criminator used in the apparatus of Figurel.

Similar reference characters. are applied to similarv elements throughout the drawing.

An output signal isr Referring to Figure l, a.- substantially constant intensity light source 11, such as an incandescent lamp, is positioned adjacent the cathode of a photomultiplier device 13 so that most of the light photon energy emitted by the source initiates electron emission from the` cathode. The electrons liberated in response to the incident light photon energy are amplilied in successive dynode stages of the electron multiplier section of the device 13. The photo, multiplier output comprises pulses having a distribution of pulse heights which (at pulse heights which are large compared to theaverage pulse height which. would be derived from a single electron) is quite close to a logarithmic curve. The photomultiplier output may be fura ther amplified, if desired, by means of a pulse amplifier 1S which is coupled to the output circuit of the device 13. The pulse output signals from the amplilier 15l are then applied to a variable bias discriminator 17, for eX- ample, as shown in Figure 2.

Referring to the discriminator illustrated inV FigureY 2, amplified photomultiplier pulses are developed across a resistor 19 in the input circuit of one 21. of two tubes 21, 23 (preferably a double-triode structure within a single envelope) which. are connected as a cathode-coupled amplifier. Tube 21 initially is cut ofi.V A given positive input pulse 25 drives the tube 21 into conduction and substantially simultaneously causes the potential devel# oped across the cathode resistor 27, common to both tubes to increase. The cathode potential increase causes the negative bias of tube 23 to increase and its respective electron current conduction to decrease. Under these conditions the voltage at the anode of tube 23 becomes more positive. TheY anodev voltage remains at this more postive value unt-il pulse 25 no longer is applied to tube 21. Tube 21 then is again in cut-off. At this time the anode voltage of tube 23 returns to its initial or nopulse value. An amplified positive going pulse 29y thus is provided at the output of the discriminator for each positive input pulse applied thereto and having an amphtude great enough to drive tube 21 into conduction. The bias applied to the grid of` tube 23 determines the pulse height threshold which input pulses such as 25 must attain before anf output pulse is derived from' the output circuit of tube 23. Means for automatically controlling the' bias will be shown below.

Referring again to Figure l, pulse signals derived-from.- the photomultiplier in' the manner described above are applied to one input circuit of a dual-input bistable multi vibrator 31. Multivibrator' 31 may be of the typ/eV illustrated at page 174 (Figures 4-8) of Ultra-I-Iigh Frequency' Techniques by Brainerd, Koehler, Reich, and Woodru (D; Van Nostrand-l.946). signals, therate of recurrence of which is to be measured, are applied to ther remaining input circuit ofthe multi-5 vibrator 31. pulses from the photomultiplier circuitry causesfthe multivibrator to be set to one of'itsl two stable conditions (state l) and pulses at the rate toA beY measured set the device to its other bistable condition (state 2).

Since the multivibrator is actuated by dual inputs andi` isset thereby to either state l` or state 2 the voltage at the output of the multivibrator varies between these limitsA The output of the D.-C. amplifier circuitV (a positive voltage) isthen fed back and appliedtorthe bias-circuit The feedback' of the variable bias discriminator 17'. voltage automatically ladjusts thebias of the: discriminator 17,:so that aconstant ratio ofphotomultiplier andvpulses:

Randomly occurring pulsel The application to the multivibrator of Preferably th'e meter, the invention is equally useful as a frequency Y measuring device. Sinusoidalwaves at the frequency to be measured may be limited and differentiated in a known manner to -obtain pulse signals. These pulse signals may then be applied to the bistable multivibrator in the same manner as herein describedwith reference to randomly occurring pulses. The output meter again is calibrated logarithmically and in this instance provides an indication of the frequency of the sinusoidal input signals.

The instrument herein described thus affords relatively simple and practical means for measuring the count rate or frequency of pulse or sinusoidal signals corresponding to nuclear or other phenomena. The instrument provides such measurement with reasonable accuracy over a relatively wide range having a ratio of counting rates of the order of 105. Also the presentV apparatus does not require undesirable scale changing.

What is claimed is:

l. An electrical measuring apparatus comprising, connection means for receiving input signals the count rate or frequency of which is to be measured, means for generating pulses having a logarithmic pulse height distribution, a signal conveying channel coupled to said pulse generating means biased to pass only pulses having pulse heights great enough to overcome said bias, means coupled to said connection means and said signal conveying channel responsive to said passed and said input signals for producing a direct-current voltage having an amplitude proportional to the ratio of pulses separately applied thereto, and feedback means for applying said direct-current voltage to said signal conveying channel for controlling the bias of said channel such that the ratio of pulses applied to said direct-current voltage producing means is constant.

2. An electrical measuring instrument comprising, connection means for receiving input signalsvthe count rate or frequency of which is to be measured, means for generating pulses having a logarithmic pulse heightV distribution, a signal conveying channel coupled to said pulse generating means including bias means forbiasing said channel to pass only pulses having pulse heights great enough to overcome said bias, means coupled to said connection means and said signal conveying channel responsive to said passed and said input signals for producing a directcurrent voltagehaving an amplitude proportional to the ratio of pulses separately applied thereto, feedback means for applying'V said direct-current voltage to said signal conveying channel "for controlling the vbias of said channel such that the ratio of pulses applied to said directcurrent voltage producing means is constant, and an indicator coupled to said feedback means for providing an indication of the amplitude of said feedback voltage.

' 3. An electrical measuring instrument comprising, connection means for receiving input signals the count rate or frequency of which is to be determined, means for generating pulses having a logarithmic pulse height distribution, a signal conveying channel coupled to said pulse generating means biased to pass only pulses having pulse heights great enough to overcome said bias, a bistable multivibrator having separate input circuits coupled to said connection means and said signal conveying channel, said multivibrator being set to a rst stable condition with an input from said source connection means applied thereto and being set to a second stable condition with an input from said signal conveying channel applied thereto, means coupled to said multivibrator for' producing a direct-current voltage having an amplitude proportional to the ratio of pulses separately applied thereto, andfeedback means for applying said direct-current voltage to said signal conveying for controlling the bias of said channel such that the ratio of pulses applied to said multivibrator is constant.

4. A measuring instrument as claimed in claim 3 including an indicator Vcoupled'to said feedback means for providing an indication of the amplitude of said feedback voltage.

5. An electrical measuring instrument comprising, connection means for receiving input signals the count' rate or frequency of which is to be measured, means including a constant intensity light ray source and a secondary emission electron multiplier responsive to said light ray source for generating pulses having a logarithmic pulse height distribution, a signal conveying channel coupled to said multiplier and biased to pass only pulses having pulseheights great enough to overcome said bias, means coupled to said connection means and said signal conveying channel responsive to said passed and said input signals for producing a direct-current voltage having an amplitude proportional to the ratio of pulses separately applied thereto andvmeans for applying said direct-current voltage to said signal Aconveying channel for controlling the bias of said channel such that the ratio of pulses applied to said direct-current voltage producing means is constant.

6. A logarithmic count rate meter comprising, connection means for receiving input signals the rate of which is tov be measured, means including a secondary emission electron multiplier for generating pulses having a logarithrnic pulse 'height distribution, a signal conveying channel coupled to said pulse generating means biased to pass only pulses having pulse heights great enough to overcome said bias, a bistable multivibrator having separate input circuits coupled to said connection means and said si-gnal conveying channel, said multivibrator being set to a iirst sta'ble condition vvith an input from said source connection means applied thereto and being set to a second stable condition with an input from said signal conveying channel applied thereto, an integrating circuit coupled to said multivibrator for producing a direct-current voltage having an amplitudeY proportional to the ratio of pulses separately applied thereto, a direct-current amplifier coupled to said integrating circuit for amplifying said directcurrent voltage, Ifeedback means for applying said ampliiied direct-.current voltage to said signal conveying channel for controlling the bias of said channel such that the ratio of pulses applied to said multivibrator is constant, and an indicator coupled to said feedback means for providing an indica-tion of the amplitude of said feed-back voltage. t

7. ,An electrical measuring apparatus comprising, connect-ion means for receiving input signals the count rate or frequency 0f which is to be measured, means for generating pulses having a logarithmic pulse height distribution, a signal conveying channel coupled to said pulse generating means biased to pass only pulses having pulse heights great enough to overcome said bias, means coupled to said connection means and said signal conveying channel responsive to said passed and said input signals forY producing Ya direct-current voltage having an amplitude proportional to the ratio of pulses separately applied thereto, and feedback means forrapplyingrsaid direct-current voltage to said signal conveying channel for control-ling the bias of saidcha-nnel such tha-t the ratio of pulses applied to said direct-current voltage producing means is constant.y

lReferences Cited in the tile of this patent UNITED STATES PATENTS 2,454,871 Gunderson Nov. 30, 1948 2,576,661 Wouters Nov. 27, 1951 2,582,831 Hester Jan. 15, 1952 2,610,303 Bell Sept. 9, 1952 2,659,011 Youmans et al. Nov. l0, 1953 wa-H 

